That’s where I think the biggest distinction is, the emperors have the strength to control a portion of the seas; however, Pirate King tiers are those who are able to rule the seas. I’m not saying it’s reserved just for old legends; it’s just that current top tiers have shown that they just aren’t strong enough to surpass the other emperors. Despite how shitty big mom looked; she was still an obstacle to Kaido who I think was the strongest of the verse up until his death.
I can agree with Kaido, Big Mom, Oden, the Admirals.
But now let's look at the others:
Shanks- The Guardian of One Piece, Can be argued as the new Roger, literally wi fi diffed an Admiral
Dragon- Most Wanted Man in the World, Has an Hidden D. Power and an uncknow DF.
Mihawk- The World Strongest Swordsman,The guy who literally refused to be an Yonkou like shit.
These three are both built different and endgame material unlike the Yonkous
Yes, they didn't show the same strenght and haki as Roger and WB, but they're the most close as that Level.